I received a book on here with a caterpillar drawn in marker on the very top of the book (pages). I think that isn't postable.

Well, actually, writing on the page edges is generally postable. Names, remainder marks, stray pen or ink or marker are all OK on page edges.

What is not OK on page edges? dirt or liquid stains, or writing that has bled through into the pages.

If there is so much writing that one could consider the book "not in generally good condition" then maybe you can RWAP.

Not sure caterpillar picture does it, though. Probably an eye of the beholder thing. (Sorry).

Is it possible to have a requester condition for page edges? I'm fine with remainder marks, but not anything else. I just added the no smelly books condition (this one reeked and I had to throw it out).

You can have an RC for anything you like. Whether people choose to work with it is another question - for example, I turn down most smell RCs as they are just too subjective.

Schuylar,  If you get one of the large gray cardboard like nail files that they use in salons but you can buy most anywhere, you can file off most marks that are on edges of books like that including peoples names pretty easily if they did go too deep.